Calmodulin as an ion channel subunit

Molecular Mechanism for Divergent Regulation of Cav1.2 Ca2+ Channels by Calmodulin and Ca2+-binding Protein-1

Ca(2+)-binding protein-1 (CaBP1) and calmodulin (CaM) are highly related Ca(2+)-binding proteins that directly interact with, and yet differentially regulate, voltage-gated Ca(2+) channels. Whereas CaM enhances inactivation of Ca(2+) currents through Ca(v)1.2 (L-type) Ca(2+) channels, CaBP1 completely prevents this process. How CaBP1 and CaM mediate such opposing effects on Ca(v)1.2 inactivation is unknown. Here, we identified molecular determinants in the alpha(1)-subunit of Ca(v)1.2 (alpha(1)1.2) that distinguish the effects of CaBP1 and CaM on inactivation. Although both proteins bind to a well characterized IQ-domain in the cytoplasmic C-terminal domain of alpha(1)1.2, mutations of the IQ-domain that significantly weakened CaM and CaBP1 binding abolished the functional effects of CaM, but not CaBP1. Pulldown binding assays revealed Ca(2+)-independent binding of CaBP1 to the N-terminal domain (NT) of alpha(1)1.2, which was in contrast to Ca(2+)-dependent binding of CaM to this region. Deletion of the NT abolished the effects of CaBP1 in prolonging Ca(v)1.2 Ca(2+) currents, but spared Ca(2+)-dependent inactivation due to CaM. We conclude that the NT and IQ-domains of alpha(1)1.2 mediate functionally distinct interactions with CaBP1 and CaM that promote conformational alterations that either stabilize or inhibit inactivation of Ca(v)1.2.

Angiotensin II inhibits bTREK-1 K+ channels in adrenocortical cells by separate Ca2+- and ATP hydrolysis-dependent mechanisms

Bovine adrenocortical cells express bTREK-1 K+ channels that set the resting membrane potential (V(m)) and couple angiotensin II (AngII) and adrenocorticotropic hormone (ACTH) receptors to membrane depolarization and corticosteroid secretion. In this study, it was discovered that AngII inhibits bTREK-1 by separate Ca2+- and ATP hydrolysis-dependent signaling pathways. When whole cell patch clamp recordings were made with pipette solutions that support activation of both Ca2+- and ATP-dependent pathways, AngII was significantly more potent and effective at inhibiting bTREK-1 and depolarizing adrenal zona fasciculata cells, than when either pathway is activated separately. External ATP also inhibited bTREK-1 through these two pathways, but ACTH displayed no Ca2+-dependent inhibition. AngII-mediated inhibition of bTREK-1 through the novel Ca2+-dependent pathway was blocked by the AT1 receptor antagonist losartan, or by including guanosine-5'-O-(2-thiodiphosphate) in the pipette solution. The Ca2+-dependent inhibition of bTREK-1 by AngII was blunted in the absence of external Ca2+ or by including the phospholipase C antagonist U73122, the inositol 1,4,5-trisphosphate receptor antagonist 2-amino-ethoxydiphenyl borate, or a calmodulin inhibitory peptide in the pipette solution. The activity of unitary bTREK-1 channels in inside-out patches from adrenal zona fasciculata cells was inhibited by application of Ca2+ (5 or 10 microM) to the cytoplasmic membrane surface. The Ca2+ ionophore ionomycin also inhibited bTREK-1 currents through channels expressed in CHO-K1 cells. These results demonstrate that AngII and selected paracrine factors that act through phospholipase C inhibit bTREK-1 in adrenocortical cells through simultaneous activation of separate Ca2+- and ATP hydrolysis-dependent signaling pathways, providing for efficient membrane depolarization. The novel Ca2+-dependent pathway is distinctive in its lack of ATP dependence, and is clearly different from the calmodulin kinase-dependent mechanism by which AngII modulates T-type Ca2+ channels in these cells.

Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C

The 5-hydroxytryptamine2A (5-HT2A) receptor is a G(q/11)-coupled serotonin receptor that activates phospholipase C and increases diacylglycerol formation. In this report, we demonstrated that calmodulin (CaM) co-immunoprecipitates with the 5-HT2A receptor in NIH-3T3 fibroblasts in an agonist-dependent manner and that the receptor contains two putative CaM binding regions. The putative CaM binding regions of the 5-HT2A receptor are localized to the second intracellular loop and carboxyl terminus. In an in vitro binding assay peptides encompassing the putative second intracellular loop (i2) and carboxyl-terminal (ct) CaM binding regions bound CaM in a Ca2+-dependent manner. The i2 peptide bound with apparent higher affinity and shifted the mobility of CaM in a nondenaturing gel shift assay. Fluorescence emission spectral analyses of dansyl-CaM showed apparent K(D) values of 65 +/- 30 nM for the i2 peptide and 168 +/- 38 nM for the ct peptide. The ct CaM-binding domain overlaps with a putative protein kinase C (PKC) site, which was readily phosphorylated by PKC in vitro. CaM binding and phosphorylation of the ct peptide were found to be antagonistic, suggesting a putative role for CaM in the regulation of 5-HT2A receptor phosphorylation and desensitization. Finally, we showed that CaM decreases 5-HT2A receptor-mediated [35S]GTPgammaS binding to NIH-3T3 cell membranes, supporting a possible role for CaM in regulating receptor-G protein coupling. These data indicate that the serotonin 5-HT2A receptor contains two high affinity CaM-binding domains that may play important roles in signaling and function.

Monitoring neural activity and [Ca2+] with genetically encoded Ca2+ indicators

Genetically encoded Ca2+ indicators (GECIs) based on fluorescent proteins (XFPs) and Ca2+-binding proteins [like calmodulin (CaM)] have great potential for the study of subcellular Ca2+ signaling and for monitoring activity in populations of neurons. However, interpreting GECI fluorescence in terms of neural activity and cytoplasmic-free Ca2+ concentration ([Ca2+]) is complicated by the nonlinear interactions between Ca2+ binding and GECI fluorescence. We have characterized GECIs in pyramidal neurons in cultured hippocampal brain slices, focusing on indicators based on circularly permuted XFPs [GCaMP (Nakai et al., 2001), Camgaroo2 (Griesbeck et al., 2001), and Inverse Pericam (Nagai et al., 2001)]. Measurements of fluorescence changes evoked by trains of action potentials revealed that GECIs have little sensitivity at low action potential frequencies compared with synthetic [Ca2+] indicators with similar affinities for Ca2+. The sensitivity of GECIs improved for high-frequency trains of action potentials, indicating that GECIs are supralinear indicators of neural activity. Simultaneous measurement of GECI fluorescence and [Ca2+] revealed supralinear relationships. We compared GECI fluorescence saturation with CaM Ca2+-dependent structural transitions. Our data suggest that GCaMP and Camgaroo2 report CaM structural transitions in the presence and absence of CaM-binding peptide, respectively.

Biochemical properties of V91G calmodulin: A calmodulin point mutation that deregulates muscle contraction in Drosophila

A mutation (Cam7) to the single endogenous calmodulin gene of Drosophila generates a mutant protein with valine 91 changed to glycine (V91G D-CaM). This mutation produces a unique pupal lethal phenotype distinct from that of a null mutation. Genetic studies indicate that the phenotype reflects deregulation of calcium fluxes within the larval muscles, leading to hypercontraction followed by muscle failure. We investigated the biochemical properties of V91G D-CaM. The effects of the mutation on free CaM are minor: Calcium binding, and overall secondary and tertiary structure are indistinguishable from those of wild type. A slight destabilization of the C-terminal domain is detectable in the calcium-free (apo-) form, and the calcium-bound (holo-) form has a somewhat lower surface hydrophobicity. These findings reinforce the indications from the in vivo work that interaction with a specific CaM target(s) underlies the mutant defects. In particular, defective regulation of ryanodine receptor (RyR) channels was indicated by genetic interaction analysis. Studies described here establish that the putative CaM binding region of the Drosophila RyR (D-RyR) binds wild-type D-CaM comparably to the equivalent CaM-RyR interactions seen for the mammalian skeletal muscle RyR channel isoform (RYR1). The V91G mutation weakens the interaction of both apo- and holo-D-CaM with this binding region, and decreases the enhancement of the calcium-binding affinity of CaM that is detectable in the presence of the RyR target peptide. The predicted functional consequences of these changes are consonant with the in vivo phenotype, and indicate that D-RyR is one, if not the major, target affected by the V91G mutation in CaM.

Modulation of skeletal and cardiac voltage-gated sodium channels by calmodulin

Calmodulin (CaM) has been shown to modulate different ion channels, including voltage-gated sodium channels (NaChs). Using the yeast two-hybrid assay, we found an interaction between CaM and the C-terminal domains of adult skeletal (NaV1.4) and cardiac (NaV1.5) muscle NaChs. Effects of CaM were studied using sodium channels transiently expressed in CHO cells. Wild type CaM (CaM(WT)) caused a hyperpolarizing shift in the voltage dependence of activation and inactivation for NaV1.4 and activation for NaV1.5. Intracellular application of CaM caused hyperpolarizing shifts equivalent to those seen with CaM(WT) coexpression with NaV1.4. Elevated Ca2+ and CaM-binding peptides caused depolarizing shifts in the inactivation curves seen with CaM(WT) coexpression with NaV1.4. KN93, a CaM-kinase II inhibitor, had no effect on NaV1.4, suggesting that CaM acts directly on NaV1.4 and not through activation of CaM-kinase II. Coexpression of hemi-mutant CaMs showed that an intact N-terminal lobe of CaM is required for effects of CaM upon NaV1.4. Mutations in the sodium channel IQ domain disrupted the effects of CaM on NaV1.4: the I1727E mutation completely blocked all calmodulin effects, while the L1736R mutation disrupted the effects of Ca2+-calmodulin on inactivation. Chimeric channels of NaV1.4 and NaV1.5 also indicated that the C-terminal domain is largely responsible for CaM effects on inactivation. CaM had little effect on NaV1.4 expressed in HEK cells, possibly due to large differences in the endogenous expression of beta-subunits between CHO and HEK cells. These results in heterologous cells suggest that Ca2+ released during muscle contraction rapidly modulates NaCh availability via CaM.

The mammalian TRPC cation channels

Transient Receptor Potential-Canonical (TRPC) channels are mammalian homologs of Transient Receptor Potential (TRP), a Ca(2+)-permeable channel involved in the phospholipase C-regulated photoreceptor activation mechanism in Drosophila. The seven mammalian TRPCs constitute a family of channels which have been proposed to function as store-operated as well as second messenger-operated channels in a variety of cell types. TRPC channels, together with other more distantly related channel families, make up the larger TRP channel superfamily. This review summarizes recent findings on the structure, regulation and function of the apparently ubiquitous TRPC cation channels.

Calmodulin mediates norepinephrine-induced receptor-operated calcium entry in preglomerular resistance arteries

Although L-type voltage-dependent calcium channels play a major role in mediating vascular smooth muscle cell contraction in the renal vasculature, non-L-type calcium entry mechanisms represent a significant component of vasoactive agonist-induced calcium entry in these cells as well. To investigate the role of these non-voltage-dependent calcium entry pathways in the regulation of renal microvascular reactivity, we have characterized the function of store- and receptor-operated channels (SOCs and ROCs) in renal cortical interlobular arteries (ILAs) of rats. Using fura 2-loaded, microdissected ILAs, we find that the L-type channel antagonist nifedipine blocks less than half the rise in intracellular calcium concentration ([Ca(2+)](i)) elicited by norepinephrine. SOCs were activated in these vessels using the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) inhibitors cyclopiazonic acid and thapsigargin and were dose dependently blocked by the SOC antagonists Gd(3+) and 2-aminoethoxydiphenyl borate (2-APB) and the combined SOC/ROC antagonist SKF-96365. Gd(3+) had no effect on the non-L-type Ca(2+) entry activated by 1 microM NE. A low concentration of SKF-96365 that did not affect thapsigargin-induced store-operated Ca(2+) entry blocked 60-70% of the NE-induced Ca(2+) entry. Two different calmodulin inhibitors (W-7 and trifluoperazine) also blocked the NE-induced Ca(2+) entry. These data suggest that in addition to L-type channels, NE primarily activates ROCs rather than SOCs in ILAs and that this receptor-operated Ca(2+) entry mechanism is regulated by calmodulin. Interestingly, 2-APB completely blocked the NE-induced non-L-type Ca(2+) entry, implying that SOCs and ROCs in preglomerular resistance vessels share a common molecular structure.

Regulation of the voltage-gated potassium channel KCNQ4 in the auditory pathway

The potassium channel KCNQ4, expressed in the mammalian cochlea, has been associated tentatively with an outer hair cell (OHC) potassium current, I(K,n), a current distinguished by an activation curve shifted to exceptionally negative potentials. Using CHO cells as a mammalian expression system, we have examined the properties of KCNQ4 channels under different phosphorylation conditions. The expressed current showed the typical KCNQ4 voltage-dependence, with a voltage for half-maximal activation (V(1/2)) of -25 mV, and was blocked almost completely by 200 microM linopirdine. Application of 8-bromo-cAMP or the catalytic sub-unit of PKA shifted V(1/2) by approximately -10 and -20 mV, respectively. Co-expression of KCNQ4 and prestin, the OHC motor protein, altered the voltage activation by a further -15 mV. Currents recorded with less than 1 nM Ca(2+) in the pipette ran down slowly (12% over 5 min). Buffering the pipette Ca(2+) to 100 nM increased the run-down rate sevenfold. Exogenous PKA in the pipette prevented the effect of elevated [Ca(2+)](i) on run-down. Inhibition of the calcium binding proteins calmodulin or calcineurin by W-7 or cyclosporin A, respectively, also prevented the calcium-dependent rapid run-down. We suggest that KCNQ4 phosphorylation via PKA and coupling to a complex that may include prestin can lead to the negative activation and the negative resting potential found in adult OHCs.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Regulation of the Voltage-Gated Cardiac Sodium Channel Nav1.5 by Interacting Proteins

Na(v)1.5, the major cardiac voltage-gated Na(+) channel, plays a central role in the generation of the cardiac action potential and in the propagation of electrical impulses in the heart. Its importance for normal heart function has been recently exemplified by reports of numerous mutations found in the gene SCN5A--which encodes Na(v)1.5--in patients with various pathologic cardiac phenotypes, indicating that even subtle alterations of Na(v)1.5 cell biology and function may underlie human diseases. Similar to other ion channels, Na(v)1.5 is most likely part of dynamic multiprotein complexes located in the different cellular compartments. This review focuses on five intracellular proteins that have been recently reported to directly bind to and contribute to the regulation of Na(v)1.5: ankyrin proteins, fibroblast growth factor homologous factor 1B, calmodulin, Nedd4-like ubiquitin-protein ligases, and syntrophin proteins.

Calmodulin reverses rundown of L-type Ca(2+) channels in guinea pig ventricular myocytes

Calmodulin (CaM) is implicated in regulation of Ca(2+) channels as a Ca(2+) sensor. The effect of CaM on rundown of L-type Ca(2+) channels in inside-out patch form was investigated in guinea pig ventricular myocytes. Ca(2+) channel activity disappeared within 1-3 min and did not reappear when the patch was excised and exposed to an artificial intracellular solution. However, application of CaM (0.03, 0.3, 3 microM) + 3 mM ATP to the intracellular solution within 1 min after patch excision resulted in dose-dependent activation of channel activity. Channel activity averaged 11.2%, 94.7%, and 292.9%, respectively, of that in cell-attached mode. Channel activity in inside-out patch mode was induced by CaM + ATP at nanomolar Ca(2+) concentrations ([Ca(2+)]); however, increase to micromolar [Ca(2+)] rapidly inactivated the channel activity induced, revealing that the effect of CaM on the channel was Ca(2+) dependent. At the 2nd, 4th, 6th, 8th, and 10th minutes after patch excision, CaM (0.75 microM) + ATP induced Ca(2+) channel activity to 150%, 100%, 96.9%, 29.3%, and 16.6%, respectively, revealing a time-dependent action of CaM on the channel. CaM added with adenosine 5'-(beta,gamma-imido)triphosphate (AMP-PNP) also induced channel activity, although with much lower potency and shorter duration. Protein kinase inhibitors KN-62, CaM-dependent protein kinase (CaMK)II 281-309, autocamtide-related CaMKII inhibitor peptide, and K252a (each 1-10 microM) did not block the effect of CaM, indicating that the effect of CaM on the Ca(2+) channel was phosphorylation independent. Neither CaM nor ATP alone induced Ca(2+) channel activity, showing a cooperative effect of CaM and ATP on the Ca(2+) channel. These results suggest that CaM is a crucial regulatory factor of Ca(2+) channel basal activity.

Goldfish calmodulin: molecular cloning, tissue distribution, and regulation of transcript expression in goldfish pituitary cells

Calmodulin (CaM) is a Ca(2+)-binding protein essential for biological functions mediated through Ca(2+)-dependent mechanisms. In the goldfish, CaM is involved in the signaling events mediating pituitary hormone secretion induced by hypothalamic factors. However, the structural identity of goldfish CaM has not been established, and the neuroendocrine mechanisms regulating CaM gene expression at the pituitary level are still unknown. Here we cloned the goldfish CaM and tested the hypothesis that pituitary expression of CaM transcripts can be the target of modulation by hypothalamic factors. Three goldfish CaM cDNAs, namely CaM-a, CaM-bS, and CaM-bL, were isolated by library screening. These cDNAs carry a 450-bp open reading frame encoding the same 149-amino acid CaM protein, the amino acid sequence of which is identical with that of mammals, birds, and amphibians and is highly homologous (>/=90%) to that in invertebrates. In goldfish pituitary cells, activation of cAMP- or PKC-dependent pathways increased CaM mRNA levels, whereas the opposite was true for induction of Ca(2+) entry. Basal levels of CaM mRNA was accentuated by GnRH and pituitary adenylate cyclase-activating polypeptide but suppressed by dopaminergic stimulation. Pharmacological studies using D1 and D2 analogs revealed that dopaminergic inhibition of CaM mRNA expression was mediated through pituitary D2 receptors. At the pituitary level, D2 activation was also effective in blocking GnRH- and pituitary adenylate cyclase-activating polypeptide-stimulated CaM mRNA expression. As a whole, the present study has confirmed that the molecular structure of CaM is highly conserved, and its mRNA expression at the pituitary level can be regulated by interactions among hypothalamic factors.

Multiple regulation by calcium of murine homologues of transient receptor potential proteins TRPC6 and TRPC7 expressed in HEK293 cells

We investigated, by using the patch clamp technique, Ca2+-mediated regulation of heterologously expressed TRPC6 and TRPC7 proteins in HEK293 cells, two closely related homologues of the transient receptor potential (TRP) family and molecular candidates for native receptor-operated Ca2+ entry channels. With nystatin-perforated recording, the magnitude and time courses of activation and inactivation of carbachol (CCh; 100 microM)-activated TRPC6 currents (I(TRPC6)) were enhanced and accelerated, respectively, by extracellular Ca2+ (Ca2+(o)) whether it was continuously present or applied after receptor stimulation. In contrast, Ca2+(o) solely inhibited TRPC7 currents (I(TRPC7)). Vigorous buffering of intracellular Ca2+ (Ca2+(i)) under conventional whole-cell clamp abolished the slow potentiating (i.e. accelerated activation) and inactivating effects of Ca2+(o), disclosing fast potentiation (EC50: approximately 0.4 mM) and inhibition (IC50: approximately 4 mM) of I(TRPC6) and fast inhibition (IC50: approximately 0.4 mM) of I(TRPC7). This inhibition of I(TRPC6) and I(TRPC7) seems to be associated with voltage-dependent reductions of unitary conductance and open probability at the single channel level, whereas the potentiation of I(TRPC6) showed little voltage dependence and was mimicked by Sr2+ but not Ba2+. The activation process of I(TRPC6) or its acceleration by Ca2+(o) probably involves phosphorylation by calmodulin (CaM)-dependent kinase II (CaMKII), as pretreatment with calmidazolium (3 microM), coexpression of Ca2+-insensitive mutant CaM, and intracellular perfusion of the non-hydrolysable ATP analogue AMP-PNP and a CaMKII-specific inhibitory peptide all effectively prevented channel activation. However, this was not observed for TRPC7. Instead, single CCh-activated TRPC7 channel activity was concentration-dependently suppressed by nanomolar Ca2+(i) via CaM and conversely enhanced by IP3. In addition, the inactivation time course of I(TRPC6) was significantly retarded by pharmacological inhibition of protein kinase C (PKC). These results collectively suggest that TRPC6 and 7 channels are multiply regulated by Ca2+ from both sides of the membrane through differential Ca2+-CaM-dependent and -independent mechanisms.

Drosophila calmodulin mutants with specific defects in the musculature or in the nervous system

We have studied lethal mutations in the single calmodulin gene (Cam) of Drosophila to gain insight into the in vivo functions of this important calcium sensor. As a result of maternal calmodulin (CaM) in the mature egg, lethality is delayed until the postembryonic stages. Prior to death in the first larval instar, Cam nulls show a striking behavioral abnormality (spontaneous backward movement) whereas a mutation, Cam7, that results in a single amino acid change (V91G) produces a very different phenotype: short indented pupal cases and pupal death with head eversion defects. We show here that the null behavioral phenotype originates in the nervous system and involves a CaM function that requires calcium binding to all four sites of the protein. Further, backward movement can be induced in hypomorphic mutants by exposure to high light levels. In contrast, the V91G mutation specifically affects the musculature and causes abnormal calcium release in response to depolarization of the muscles. Genetic interaction studies suggest that failed regulation of the muscle calcium release channel, the ryanodine receptor, is the major defect underlying the Cam7 phenotype.

Ca2+-activated K+ channels: molecular determinants and function of the SK family

Ca(2+)-activated K(+) (K(Ca)) channels of small (SK) and intermediate (IK) conductance are present in a wide range of excitable and non-excitable cells. On activation by low concentrations of Ca(2+), they open, which results in hyperpolarization of the membrane potential and changes in cellular excitability. K(Ca)-channel activation also counteracts further increases in intracellular Ca(2+), thereby regulating the concentration of this ubiquitous intracellular messenger in space and time. K(Ca) channels have various functions, including the regulation of neuronal firing properties, blood flow and cell proliferation. The cloning of SK and IK channels has prompted investigations into their gating, pharmacology and organization into calcium-signalling domains, and has provided a framework that can be used to correlate molecularly identified K(Ca) channels with their native currents.

Ca2+-mediated Potentiation of the Swelling-induced Taurine Efflux from HeLa Cells: On the Role of Calmodulin and Novel Protein Kinase C Isoforms

The present work sets out to investigate how Ca(2+) regulates the volume-sensitive taurine-release pathway in HeLa cells. Addition of Ca(2+)-mobilizing agonists at the time of exposure to hypotonic NaCl medium augments the swelling-induced taurine release and subsequently accelerates the inactivation of the release pathway. The accelerated inactivation is not observed in hypotonic Ca(2+)-free or high-K(+) media. Addition of Ca(2+)-mobilizing agonists also accelerates the regulatory volume decrease, which probably reflects activation of Ca(2+)-activated K(+) channels. The taurine release from control cells and cells exposed to Ca(2+) agonists is equally affected by changes in cell volume, application of DIDS and arachidonic acid, indicating that the volume-sensitive taurine leak pathway mediates the Ca(2+)-augmented taurine release. Exposure to Ca(2+)-mobilizing agonists prior to a hypotonic challenge also augments a subsequent swelling-induced taurine release even though the intracellular Ca(2+)-concentration has returned to the unstimulated level. The Ca(2+)-induced augmentation of the swelling-induced taurine release is abolished by inhibition of calmodulin, but unaffected by inhibition of calmodulin-dependent kinase II, myosin light chain kinase and calcineurin. The effect of Ca(2+)-mobilizing agonists is mimicked by protein kinase C (PKC) activation and abolished in the presence of the PKC inhibitor Gö6850 and following downregulation of phorbol ester-sensitive PKC isoforms. It is suggested that Ca(2+) regulates the volume-sensitive taurine-release pathway through activation of calmodulin and PKC isoforms belonging to the novel subclass (nPKC).

In intact mammalian photoreceptors, Ca2+-dependent modulation of cGMP-gated ion channels is detectable in cones but not in rods

In the mammalian retina, cone photoreceptors efficiently adapt to changing background light intensity and, therefore, are able to signal small differences in luminance between objects and backgrounds, even when the absolute intensity of the background changes over five to six orders of magnitude. Mammalian rod photoreceptors, in contrast, adapt very little and only at intensities that nearly saturate the amplitude of their photoresponse. In search of a molecular explanation for this observation we assessed Ca²⁺-dependent modulation of ligand sensitivity in cyclic GMP–gated (CNG) ion channels of intact mammalian rods and cones. Solitary photoreceptors were isolated by gentle proteolysis of ground squirrel retina. Rods and cones were distinguished by whether or not their outer segments bind PNA lectin. We measured membrane currents under voltage-clamp in photoreceptors loaded with Diazo-2, a caged Ca²⁺ chelator, and fixed concentrations of 8Br-cGMP. At 600 nM free cytoplasmic Ca²⁺ the midpoint of the cone CNG channels sensitivity to 8BrcGMP, ^8BrcGMPK_1/2, is ∼2.3 μM. The ligand sensitivity is less in rod than in cone channels. Instantly decreasing cytoplasmic Ca²⁺ to <30 nM activates a large inward membrane current in cones, but not in rods. Current activation arises from a Ca²⁺ -dependent modulation of cone CNG channels, presumably because of an increase in their affinity to the cyclic nucleotide. The time course of current activation is temperature dependent; it is well described by a single exponential process of ∼480 ms time constant at 20–21°C and 138 ms at 32°C. The absence of detectable Ca²⁺-dependent CNG current modulation in intact rods, in view of the known channel modulation by calmodulin in-vitro, affirms the modulation in intact rods may only occur at low Ca²⁺ concentrations, those expected at intensities that nearly saturate the rod photoresponse. The correspondence between Ca²⁺ dependence of CNG modulation and the ability to light adapt suggest these events are correlated in photoreceptors.

Ca2+/calmodulin modulates TRPV1 activation by capsaicin

TRPV1 ion channels mediate the response to painful heat, extracellular acidosis, and capsaicin, the pungent extract from plants in the Capsicum family (hot chili peppers) (Szallasi, A., and P.M. Blumberg. 1999. Pharmacol. Rev. 51:159–212; Caterina, M.J., and D. Julius. 2001. Annu. Rev. Neurosci. 24:487–517). The convergence of these stimuli on TRPV1 channels expressed in peripheral sensory nerves underlies the common perceptual experience of pain due to hot temperatures, tissue damage and exposure to capsaicin. TRPV1 channels are nonselective cation channels (Caterina, M.J., M.A. Schumacher, M. Tominaga, T.A. Rosen, J.D. Levine, and D. Julius. 1997. Nature. 389:816–824). When activated, they produce depolarization through the influx of Na⁺, but their high Ca²⁺ permeability is also important for mediating the response to pain. In particular, Ca²⁺ influx is thought to be required for the desensitization to painful sensations over time (Cholewinski, A., G.M. Burgess, and S. Bevan. 1993. Neuroscience. 55:1015–1023; Koplas, P.A., R.L. Rosenberg, and G.S. Oxford. 1997. J. Neurosci. 17:3525–3537). Here we show that in inside-out excised patches from TRPV1 expressed in Xenopus oocytes and HEK 293 cells, Ca²⁺/calmodulin decreased the capsaicin-activated current. This inhibition was not mimicked by Mg²⁺, reflected a decrease in open probability, and was slowly reversible. Furthermore, increasing the calmodulin concentration in our patches by coexpression of wild-type calmodulin with TRPV1 produced inhibition by Ca²⁺ alone. In contrast, patches excised from cells coexpressing TRPV1 with a mutant calmodulin did not respond to Ca²⁺. Using an in vitro calmodulin-binding assay, we found that TRPV1 in oocyte lysates bound calmodulin, although in a Ca²⁺-independent manner. Experiments with GST-fusion proteins corresponding to regions of the channel NH₂-terminal domain demonstrated that a stretch of ∼30 amino acids adjacent to the first ankyrin repeat bound calmodulin in a Ca²⁺-dependent manner. The physiological response to pain involves an influx of Ca²⁺ through TRPV1. Our results indicate that this Ca²⁺ influx may feed back on the channels, inhibiting their gating. This type of feedback inhibition could play a role in the desensitization produced by capsaicin.

Ca2+/calmodulin-dependent protein kinase II potentiates ATP responses by promoting trafficking of P2X receptors

To elucidate the functional link between Ca(2+)/calmodulin protein kinase II (CaMKII) and P2X receptor activation, we studied the effects of electrical stimulation, such as occurs in injurious conditions, on P2X receptor-mediated ATP responses in primary sensory dorsal root ganglion neurons. We found that endogenously active CaMKII up-regulates basal P2X3 receptor activity in dorsal root ganglion neurons. Electrical stimulation causes prolonged increases in ATP currents that lasts up to approximately 45 min. In addition, the total and phosphorylated CaMKII are also up-regulated. The enhancement of ATP currents depends on Ca(2+) and calmodulin and is completely blocked by the CaMKII inhibitor, 2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine). Western analyses indicate that electrical stimulation enhances the expression of P2X3 receptors in the membrane and that the enhancement is blocked by the inhibitor. These results suggest that CaMKII up-regulated by electrical stimulation enhances ATP responses by promoting trafficking of P2X receptors to the membrane and may play a key role in the sensitization of P2X receptors under injurious conditions.

Direct effect of Ca2+-calmodulin on cGMP-activated Ca2+-dependent Cl-channels in rat mesenteric artery myocytes

Recently a novel cGMP-activated Ca2+-dependent Cl- channel has been described in rat mesenteric artery smooth muscle cells. In the present work we have investigated the actions of calmodulin (CaM) on single channel cGMP-activated Ca2+-dependent Cl- current (ICl(cGMP,Ca) in inside-out patches. When 1 microm CaM was applied to the intracellular surface of inside-out patches bathed with 10 microm cGMP and 100 nm [Ca2+]i there was approximately a 10-fold increase in channel open probability (NPo). This effect of CaM was not observed with lower [Ca2+]i and 100 nm [Ca2+]i with 1 microm CaM did not activate Cl- channels in the absence of cGMP. The unitary conductance, reversal potential and mean open time of the single-channel currents were similar in the absence or presence of CaM. With 10 microm cGMP and 100 nm [Ca2+]i the relationship between NPo and CaM concentration was well fitted by the Hill equation yielding an equilibrium constant for CaM of about 1.9 nm and a Hill coefficient of 1.7. With 1 microm CaM (+10 microm cGMP) the relationship between [Ca2+]i and NPo was also fitted by the Hill equation which yielded an apparent equilibrium constant of 74 nm [Ca2+]i and a Hill coefficient of 4.8. When [Ca2+]i was increased from 300 nm to 1 microm there was a decrease in NPo. The potentiating effect of CaM was markedly reduced by the selective CaM binding peptide Trp (5 nm) but not by the Ca2+/CaM-dependent protein kinase II (CaMKII) inhibitor autocamtide II related inhibitory peptide (AIP). It is concluded that CaM potentiates the activity of single channel ICl(cGMP,Ca) by increasing the probability of channel opening via a CaMKII-independent mechanism.

Cloning and expression of a pivotal calcium metabolism regulator: calmodulin involved in shell formation from pearl oyster (Pinctada fucata)

The shells of bivalves are mainly composed of calcium carbonate, a product of calcium metabolism. In the process of shell formation, the uptake, transport and recruitment of calcium ion are highly regulated and involved in many factors. Among these regulatory factors, calmodulin (CaM), a pivotal multifunction regulator of calcium metabolism in nearly all organisms, is thought to play an important role in the calcium metabolism involved in shell formation. In this study, a full-length CaM cDNA was isolated from the pearl oyster (Pinctada fucata). The oyster calmodulin encodes a 16.8 kDa protein which shares high similarity with vertebrate calmodulin. The oyster CaM mRNA shows the highest level of expression in the gill, a key organ involved in calcium uptake in oyster calcium metabolism. In situ hybridization results revealed that oyster CaM mRNA is expressed at the folds and the outer epithelial cells of the dorsal region of the mantle, suggesting that CaM is involved in regulation of calcium transport and secretion. Oyster CaM also showed a typical Ca2+ dependent electrophoretic shift characterization and calcium binding activity. Taken together, we have identified and characterized a pivotal calcium metabolism regulator of the oyster that may play an important role in regulation of calcium uptake, transport and secretion in the process of shell formation.

Cellular mechanisms in sympatho-modulation of the heart

Cardiovascular function relies on complex servo-controlled regulation mechanisms that involve both fast-acting feedback responses and long-lasting adaptations affecting the gene expression. The adrenergic system, with its specific receptor subtypes and intracellular signalling cascades provides the major regulatory system, while the parasympathetic system plays a minor role. At the molecular level, Ca(2+) acts as the general signal trigger for the majority of cell activities including contraction, metabolism and growth. During recent years, important new results have emerged allowing an integrated view of how the multifarious Ca(2+)-signalling mechanisms transmit adrenergic impulses to intracellular target sites. These insights into cellular and molecular mechanisms are pivotal in improving pharmacological control of the sympathetic responses to surgical trauma and perioperative stress. They are examined in detail in this review, with particular emphasis being given to the differences in intracellular signalling between cardiomyocytes and vascular smooth muscle cells.

Different regions in skeletal and cardiac muscle ryanodine receptors are involved in transducing the functional effects of calmodulin

Calmodulin (CaM) inhibits the skeletal muscle ryanodine receptor-1 (RyR1) and cardiac muscle RyR2 at micromolar Ca(2+) but activates RyR1 and inhibits RyR2 at submicromolar Ca(2+) by binding to a single, highly conserved CaM-binding site. To identify regions responsible for the differential regulation of RyR1 and RyR2 by CaM, we generated chimeras encompassing and flanking the CaM-binding domain. We found that the exchange of the N- and C-terminal flanking regions differentially affected RyR1 and RyR2. A RyR1/RyR2 chimera with an N-terminal flanking RyR2 substitution (RyR2 amino acid (aa) 3537-3579) was activated by CaM in single channel measurements at both submicromolar and micromolar Ca(2+). A RyR2/RyR1 chimera with a C-terminal flanking the 86-amino acid RyR1 substitution (RyR1 aa 3640-3725) bound (35)S-CaM but was not inhibited by CaM at submicromolar Ca(2+). In this region, five non-conserved amino acid residues (RyR1 aa 3680 and 3682-3685 and RyR2 aa 3647 and 3649-3652) differentially affect RyR helical probability. Substitution of the five amino acid residues in RyR1 with those of RyR2 showed responses to CaM comparable with wild type RyR1. In contrast, substitution of the five amino acid residues in RyR2 with those of RyR1 showed loss of CaM inhibition, whereas substitution of the five RyR2 sequence residues in the RyR2 chimera containing the RyR1 calmodulin-binding domain and C-flanking sequence restored wild type RyR2 inhibition by CaM at submicromolar Ca(2+). The results suggest that different regions are involved in CaM modulation of RyR1 and RyR2. They further suggest that five non-conserved amino acids in the C-terminal region flanking the CaM-binding domain have a key role in CaM inhibition of RyR2.

Regulation of calcium signaling by polycystin-2

Autosomal dominant PKD (ADPKD) is a common lethal genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other target organs. ADPKD is caused by mutations in the PKD1 and PKD2 genes, encoding the transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Although the function and putative interacting ligands of PC1 are largely unknown, recent evidence indicates that PC2 behaves as a TRP-type Ca2+-permeable nonselective cation channel. The PC2 channel is implicated in the transient increase in cytosolic Ca2+in renal epithelial cells and may be linked to the activation of subsequent signaling pathways. Recent studies also indicate that PC1 functionally interacts with PC2 such that the PC1-PC2 channel complex is an obligatory novel signaling pathway implicated in the transduction of environmental signals into cellular events. The present review purposely avoids issues of regulation of PC2 expression and trafficking and focuses instead on the evidence for the TRP-type cation channel function of PC2. How its role as a cation channel may unmask mechanisms that trigger Ca2+transport and regulation is the focus of attention. PC2 channel function may be essential in renal cell function and kidney development. Nonrenal-targeted expression of PC2 and related proteins, including the cardiovascular system, also suggests previously unforeseeable roles in signal transduction.

Calmodulin-binding proteins in bryophytes: identification of abscisic acid-, cold-, and osmotic stress-induced genes encoding novel membrane-bound transporter-like proteins

Plant responses to environmental stresses are mediated in part by signaling processes involving cytosolic Ca2+ and a Ca(2+)-binding protein, calmodulin. Screening with radiolabeled calmodulin of a cDNA library of the moss Physcomitrella patens resulted in identification of genes encoding novel membrane transporter-like proteins, MCamb1 and MCamb2. These proteins each had a central hydrophobic domain with two putative membrane spans and N- and C-terminal hydrophilic domains, and showed sequence similarity to mammalian inward rectifier potassium channels. Calmodulin binds to MCamb1 and MCamb2 via interaction with basic amphiphilic amino acids in the C-terminal domain. Levels of MCamb1 and MCamb2 transcripts increased dramatically following treatment with low temperature, hyperosmotic solutes, and the stress hormone abscisic acid, all of which were previously shown to increase cellular tolerance to freezing stress. These results suggest that calmodulin participates in cellular signaling events leading to enhancement of stress resistance through regulation of novel transporter-like proteins.

Proteolytic cleavage of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) is regulated by a calmodulin-binding motif

Homophilic engagement of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) induces 'outside-in' signal transduction that results in phosphorylation events and recruitment and activation of signalling molecules. The formation of signalling scaffolds with PECAM-1 are important signalling events that modulate platelet secretion, aggregation and platelet thrombus formation. In this study, we describe a novel interaction between PECAM-1 and cytosolic calmodulin (CaM) in platelets. Reciprocal co-immunoprecipitation studies revealed that cytosolic CaM is constitutively associated with PECAM-1 in resting, thrombin activated and aggregated human platelets. Our studies demonstrate that CaM directly interacts with a PECAM-1 peptide (594-604) C595A containing the sequences (594)KAFYLRKAKAK(604). This CaM:PECAM-1 interaction has a threefold higher affinity than CaM:GPVI interaction. It is potentiated by the addition of calcium ions, and dissociated by the CaM inhibitor, trifluoperazine. Treatment of platelets with CaM inhibitors triggers cleavage of PECAM-1 in a time- and dose-dependent manner. Furthermore, this membrane proximal portion of PECAM-1 is conserved across mammalian species and the helical representation of basic/hydrophobic residues reveals a charge distribution analogous to other CaM-binding motifs in other proteins. Taken together, these results suggest that this highly charged cluster of amino acids in the PECAM-1 cytoplasmic domain directly interacts with CaM and this novel interaction appears to regulate cleavage of PECAM-1.

Chemical gating of gap junction channels; roles of calcium, pH and calmodulin

Both Ca(2+) and H(+) play a role in chemical gating of gap junction channels, but, with the possible exception of Cx46 hemichannels, neither of them is likely to induce gating by a direct interaction with connexins. Some evidence suggests that low pH(i) affects gating via an increase in [Ca(2+)](i); in turn, Ca(2+) is likely to induce gating by activation of CaM, which may act directly as a gating particle. The effective concentrations of both Ca(2+) and H(+) vary depending on cell type, type of connexin expressed and procedure employed to increase their cytosolic concentrations; however, pH(i) as high as 7.2 and [Ca(2+)](i) as low as 150 nM or lower have been reported to be effective in some cells. Some data suggest that Ca(2+) and H(+) affect gating by acting synergistically, but other data do not support synergism. Chemical gating follows the activation of a slow gate distinct from the fast V(j)-sensitive gate, and there is evidence that the chemical/slow gate is V(j)-sensitive. At the single channel level, the chemical/slow gate closes the channels slowly and completely, whereas the fast V(j) gate closes the channels rapidly and incompletely. At least three molecular models of channel gating have been proposed, but all of them are mostly based on circumstantial evidence.

Calcium negatively modulates calmodulin interaction with IQGAP1

IQGAP1 regulates cytoskeletal dynamics through interactions with the Rho family GTPases Rac1 and Cdc42, F-actin, and beta-catenin. Calmodulin interaction with IQ motifs of IQGAP1 negatively influences these IQGAP1 interactions. Although, calmodulin interacts with IQGAP1 in the absence of Ca(2+) and was suggested to exhibit reduced binding when Ca(2+) bound, recent reports show substantially greater binding when Ca(2+) is present. Binding evaluations have primarily relied on IQGAP1 interaction with calmodulin conjugated to Sepharose 4B. In this study we evaluated the Ca(2+)-dependence of calmodulin interaction with native IQGAP1 using a series of independent biochemical approaches. We found the apparent binding of calmodulin to IQGAP1 was Ca(2+)-independent, being between 5- and 20-fold greater in the absence than in the presence of Ca(2+). In addition, calmodulin interaction with IQGAP1 was negatively regulated by buffer [Ca(2+)] (IC(50)=3.4x10(-7)M). Regulation was specific to Ca(2+), as Ba(2+) was approximately 400-fold less effective than Ca(2+) at modulating the interaction. Moreover, testing of calmodulin mutants demonstrated that apocalmodulin tightly binds IQGAP1 and that the N- and C-terminal pair of EF hands are important for Ca(2+) sensitivity. These data indicate that calmodulin may disassemble from IQGAP1 to facilitate IQGAP1 interaction with effectors of cytoskeletal reorganization during conditions of cell activation that promote increased cytosolic [Ca(2+)].

Fast Ca(2+)-dependent inactivation of the store-operated Ca2+ current (ISOC) in liver cells: a role for calmodulin

Store-operated Ca2+ channels (SOCs) provide a major pathway for Ca2+ entry in non-excitable cells. SOCs in immortalized liver cells are highly selective for Ca2+ over other cations and are similar to well-studied Ca2+ release activated Ca2+ (CRAC) channels in haematopoietic cell lines. In the present work, employing H4IIE liver cells, we investigated fast inactivation of SOC current (ISOC), which occurs at membrane potentials below -60 mV. This inactivation was significantly reduced when BAPTA, a faster Ca2+ buffer, was used instead of EGTA, and was completely abolished if Na+ was used as a charge carrier in the absence of divalent cations in the external medium. These results suggested that fast inactivation of SOCs in H4IIE cells was Ca2+ dependent and was similar to the fast inactivation of CRAC channels. Experiments showing that the fast inactivation of ISOC was not affected by the disruption of actin by latrunculin B indicate that the cytoskeleton is unlikely to be involved. To elucidate the mechanism of Ca2+ dependence, a possible role of calmodulin (CaM) in SOCs' fast inactivation was investigated. The CaM inhibitors Mas-7 and calmidazolium failed to affect ISOC fast inactivation, whereas over-expression of a CaM inhibitor peptide or a mutant CaM lacking functional EF hands significantly altered the inactivation of ISOC. Out of two exponential components normally required to approximate kinetics of ISOC fast inactivation, the faster component was reduced in amplitude by 30%, compared to the control. The results presented suggest that CaM is responsible for at least part of Ca(2+)-dependent fast inactivation of ISOC in liver cells. It is hypothesized that CaM is tethered to the channel itself and therefore protected from chemical inhibitors.

Genetically Encoded Indicators of Cellular Calcium Dynamics Based on Troponin C and Green Fluorescent Protein

Genetic calcium probes offer tremendous potential in the fields of neuroscience, cell biology, and pharmaceutical screening. Previously, ratiometric and non-ratiometric indicators of cellular calcium dynamics have been described that consist of mutants of the green fluorescent protein (GFP) as fluorophores and calmodulin as calcium-binding moiety in several configurations. However, these calmodulin-based types of probes have a series of deficiencies, such as reduced dynamic ranges, when expressed within transgenic organisms and lack of calcium sensitivity in certain targetings. We developed novel types of calcium probes based on troponin C variants from skeletal and cardiac muscle. These indicators have ratio changes up to 140%, K(d)s ranging from 470 nm to 29 microm, and improved subcellular targeting properties. We targeted the indicators to the plasma membrane of HEK293 cells and primary hippocampal neurons. Upon long lasting depolarization, submembrane calcium levels in hippocampal neurons were found to be in equilibrium with bulk cytosolic calcium levels, suggesting no standing gradient persists from the membrane toward the cytosol. We expect that such novel indicators using specialized calcium sensing proteins will be minimally interacting with the cellular biochemical machinery.

Diversity in protein–protein interactions of connexins: emerging roles

Gap junctions, specialised membrane structures that mediate cell-to-cell communication in almost all tissues, are composed of channel-forming integral membrane proteins termed connexins. The activity of these intercellular channels is closely regulated, particularly by intramolecular modifications as phosphorylations of proteins by protein kinases, which appear to regulate the gap junction at several levels, including assembly of channels in the plasma membrane, connexin turnover as well as directly affecting the opening and closure ("gating") of channels. The regulation of membrane channels by protein phosphorylation/dephosphorylation processes commonly requires the formation of a multiprotein complex, where pore-forming subunits bind to auxiliary proteins (e.g. scaffolding proteins, catalytic and regulatory subunits), that play essential roles in channel localisation and activity, linking signalling enzymes, substrates and effectors into a structure frequently anchored to the cytoskeleton. The present review summarises the up-to-date progress regarding the proteins capable of interacting or at least of co-localising with connexins and their functional importance.

Regulation of gene expression by dietary Ca2+ in kidneys of 25-hydroxyvitamin D3-1α-hydroxylase knockout mice

BACKGROUND

Pseudovitamin D deficiency rickets (PDDR) is an autosomal disease, characterized by undetectable levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), rickets and secondary hyperparathyroidism. Mice in which the 25-hydroxyvitamin D3-1 alpha-hydroxylase (1 alpha-OHase) gene was inactivated, presented the same clinical phenotype as patients with PDDR.

METHODS

cDNA Microarray technology was used on kidneys of 1 alpha-OHase knockout mice to study the expression profile of renal genes in this Ca2+-related disorder. Genome wide molecular events that occur during the rescue of these mice by high dietary Ca2+ intake were studied by the use of 15K cDNA microarray chips.

RESULTS

1 alpha-OHase knockout mice fed a normal Ca2+ diet developed severe hypocalcemia, rickets and died with an average life span of 12 +/- 2 weeks. Intriguingly, 1 alpha-OHase-/- mice supplemented with an enriched Ca2+ diet were normocalcemic and not significantly different from wild-type mice. Inactivation of the 1 alpha-OHase gene resulted in a significant regulation of +/- 1000 genes, whereas dietary Ca2+ supplementation of the 1 alpha-OHase-/- mice revealed +/- 2000 controlled genes. Interestingly, 557 transcripts were regulated in both situations implicating the involvement in the dietary Ca2+-mediated rescue mechanism of the 1 alpha-OHase-/- mice. Conspicuous regulated genes encoded for signaling molecules like the PDZ-domain containing protein channel interacting protein, FK binding protein type 4, kinases, and importantly Ca2+ transporting proteins including the Na+-Ca2+ exchanger, calbindin-D28K and the Ca2+ sensor calmodulin.

CONCLUSION

Dietary Ca2+ intake normalized disturbances in the Ca2+ homeostasis due to vitamin D deficiency that were accompanied by the regulation of a subset of renal genes, including well-known renal Ca2+ transport protein genes, but also genes not previously identified as playing a role in renal Ca2+ handling.

Apo-Calmodulin Binds with its C-terminal Domain to the N-Methyl-d-aspartate Receptor NR1 C0 Region

Calmodulin (CaM) is the major Ca2+ sensor in eukaryotic cells. It consists of four EF-hand Ca2+ binding motifs, two in its N-terminal domain and two in its C-terminal domain. Through a negative feedback loop, CaM inhibits Ca2+ influx through N-methyl-D-aspartate-type glutamate receptors in neurons by binding to the C0 region in the cytosolic tail of the NR1 subunit. Ca2+ -depleted (apo)CaM is pre-associated with a variety of ion channels for fast and effective regulation of channel activities upon Ca2+ influx. Using the NR1 C0 region for fluorescence and circular dichroism spectroscopy studies we found that not only Ca2+ -saturated CaM but also apoCaM bound to NR1 C0. In vitro interaction assays showed that apoCaM also binds specifically to full-length NR1 solubilized from rat brain and to the complete C terminus of the NR1 splice form that contains the C0 plus C2' domain. The Ca2+ -independent interaction of CaM was also observed with the isolated C-but not N-terminal fragment of calmodulin in the independent spectroscopic assays. Fluorescence polarization studies indicated that apoCaM associated via its C-terminal domain with NR1 C0 in an extended conformation and collapsed to adopt a more compact conformation of faster rotational mobility in its complex with NR1 C0 upon addition of Ca2+. Our results indicate that apoCaM is associated with NR1 and that the complex of CaM bound to NR1 C0 undergoes a dramatic conformational change when Ca2+ binds to CaM.

Molecular determinants of Ca(2+)/calmodulin-dependent regulation of Ca(v)2.1 channels

Ca2+-dependent facilitation and inactivation (CDF and CDI) of Cav2.1 channels modulate presynaptic P/Q-type Ca2+ currents and contribute to activity-dependent synaptic plasticity. This dual feedback regulation by Ca2+ involves calmodulin (CaM) binding to the alpha1 subunit (alpha12.1). The molecular determinants for Ca2+-dependent modulation of Cav2.1 channels reside in CaM and in two CaM-binding sites in the C-terminal domain of alpha12.1, the CaM-binding domain (CBD) and the IQ-like domain. In transfected tsA-201 cells, CDF and CDI were both reduced by deletion of CBD. In contrast, alanine substitution of the first two residues of the IQ-like domain (IM-AA) completely prevented CDF but had little effect on CDI, and glutamate substitutions (IM-EE) greatly accelerated voltage-dependent inactivation but did not prevent CDI. Mutational analyses of the Ca2+ binding sites of CaM showed that both the N- and C-terminal lobes of CaM were required for full development of facilitation, but only the N-terminal lobe was essential for CDI. In biochemical assays, CaM12 and CaM34 were unable to bind CBD, whereas CaM34 but not CaM12 retained Ca2+-dependent binding to the IQ-like domain. These findings support a model in which Ca2+ binding to the C-terminal EF-hands of preassociated CaM initiates CDF via interaction with the IQ-like domain. Further Ca2+ binding to the N-terminal EF-hands promotes secondary CaM interactions with CBD, which enhance facilitation and cause a conformational change that initiates CDI. This multifaceted mechanism allows positive regulation of Cav2.1 in response to local Ca2+ increases (CDF) and negative regulation during more global Ca2+ increases (CDI).

Low-voltage triggering of Ca2+ release from the sarcoplasmic reticulum in cardiac muscle cells

This study investigated the interaction between L-type Ca2+ current (ICaL) and Ca2+ release from the sarcoplasmic reticulum (SRCR) in whole cell voltage-clamped guinea pig ventricular myocytes. Quasiphysiological cation solutions (Nao+:KI+) were used for most experiments. In control conditions, there was no obvious interaction between ICaL and SRCR. In isoproterenol, activation of ICaL from voltages between -70 and -50 mV reduced the amplitude and accelerated the decay of the current. Short (50 ms), small-amplitude voltage steps applied 60 or 510 ms before stimulating ICaL inhibited and facilitated the current, respectively. These changes were blocked by ryanodine. Low-voltage activated currents such as T-type Ca2+ current, TTX-sensitive ICa (ICaTTX), or "slip mode" Ca2+ conductance via INa+ were not responsible for low-voltage SRCR. However, L-type Ca2+ currents could be distinguished at voltages as negative as -45 mV. It is concluded that in the presence of isoproterenol, Ca2+ release from the SR at negative potentials is due to activation of L-type Ca2+ channels.

Calmodulin regulates synaptic plasticity in the anterior cingulate cortex and behavioral responses: a microelectroporation study in adult rodents

We developed a microelectroporation method for the transfer of genes into neurons in the cerebral cortex of adult rodents, both rats and mice. We selectively expressed either green-fluorescent protein (GFP) or a Ca2+-binding deficient calmodulin (CaM) mutant in the anterior cingulate cortex (ACC). In mice that expressed GFP, positive neuronal cell bodies were found specifically at the injection site in the ACC. Mice that expressed CaM12, a mutant CaM with two impaired Ca2+ binding sites in the N-terminal lobe, exhibited significant changes in vocalization, locomotion, and sensory functions. Long-term potentiation and long-term depression, two major forms of central plasticity, were completely abolished by expression of CaM12. Mice that expressed CaM34, a mutant CaM with two impaired Ca2+ binding sites in the C-terminal lobe, did not show any significant behavioral or electrophysiological alterations. These findings provide strong evidence that CaM is critical for bidirectional synaptic plasticity. This new method will be useful for investigating gene function in specific brain regions of freely moving animals. Furthermore, this approach also may facilitate gene therapy in adult human brains.

Calmodulin binds to the C terminus of sodium channels Nav1.4 and Nav1.6 and differentially modulates their functional properties

Modulation of voltage-gated sodium channels (VGSC) can have a major impact on cell excitability. Analysis of calmodulin (CaM) binding to GST-fusion proteins containing the C-terminal domains of Nav1.1-Nav1.9 indicates that some of the tetrodotoxin-sensitive VGSC isoforms, including NaV1.4 and NaV1.6, are able to bind CaM in a calcium-independent manner. Here we demonstrate that association with CaM is important for functional expression of NaV1.4 and NaV1.6 VGSCs. Disrupting the interaction between CaM and the C terminus of NaV1.4 and NaV1.6 channels reduced current amplitude by 99 and 62%, respectively. Overexpression of CaM increased the current generated by Nav1.4 and Nav1.6 C-terminal mutant constructs that exhibited intermediate current densities and intermediate binding affinities for CaM, demonstrating that this effect on current density was directly dependent on the ability of the C terminus to bind CaM. In addition to the effects on current density, calmodulin also was able to modulate the inactivation kinetics of Nav1.6, but not Nav1.4, currents in a calcium-dependent manner. Our data demonstrate that CaM can regulate the properties of VGSCs via calcium-dependent and calcium-independent mechanisms and suggest that modulation of neuronal sodium channels may play a role in calcium-dependent neuronal plasticity.

Apocalmodulin and Ca2+ calmodulin-binding sites on the CaV1.2 channel

The cardiac L-type voltage-dependent calcium channel is responsible for initiating excitation-contraction coupling. Three sequences (amino acids 1609-1628, 1627-1652, and 1665-1685, designated A, C, and IQ, respectively) of its alpha(1) subunit contribute to calmodulin (CaM) binding and Ca(2+)-dependent inactivation. Peptides matching the A, C, and IQ sequences all bind Ca(2+)CaM. Longer peptides representing A plus C (A-C) or C plus IQ (C-IQ) bind only a single molecule of Ca(2+)CaM. Apocalmodulin (ApoCaM) binds with low affinity to the IQ peptide and with higher affinity to the C-IQ peptide. Binding to the IQ and C peptides increases the Ca(2+) affinity of the C-lobe of CaM, but only the IQ peptide alters the Ca(2+) affinity of the N-lobe. Conversion of the isoleucine and glutamine residues of the IQ motif to alanines in the channel destroys inactivation (Zühlke et al., 2000). The double mutation in the peptide reduces the interaction with apoCaM. A mutant CaM unable to bind Ca(2+) at sites 3 and 4 (which abolishes the ability of CaM to inactivate the channel) binds to the IQ, but not to the C or A peptide. Our data are consistent with a model in which apoCaM binding to the region around the IQ motif is necessary for the rapid binding of Ca(2+) to the C-lobe of CaM. Upon Ca(2+) binding, this lobe is likely to engage the A-C region.

C terminus L-type Ca2+ channel calmodulin-binding domains are 'auto-agonist' ligands in rabbit ventricular myocytes

L-type Ca2+ channel C terminus calmodulin (CaM)-binding domains are molecular determinants for Ca(2+)-CaM-dependent increases in L-type Ca2+ current (ICa), and a CaM-binding IQ domain mimetic peptide (IQmp) increases L-type Ca2+ channel current by promoting a gating mode with prolonged openings (mode 2), suggesting the intriguing possibility that CaM-binding domains are 'auto-agonist' signalling molecules. In order to test the breadth of this concept, we studied the effect of a second C terminus CaM-binding domain (CB) mp (CBmp), in conjunction with IQmp, on single L-type Ca2+ channel currents in excised cell membrane patches from rabbit ventricular myocytes. Here we show that both CBmp and IQmp are agonist ligands that non-additively increase L-type Ca2+ channel opening probability (Po) by inducing mode 2 gating. CBmp and IQmp agonist effects were lost under conditions favouring calcification of CaM (Ca(2+)-CaM, 150 nM free Ca2+ and 10-20 microM CaM), but persisted in the presence of CaM (0-20 microM) under conditions adverse to Ca(2+)-CaM (20 mM BAPTA), indicating that CaM-binding domains increase L-type Ca2+ channel Po by a low Ca(2+)-CaM activity mechanism. Increasing Ca(2+)-CaM in the bath (cytosol) reduced the efficacy of CBmp and IQmp signals with Ba2+ as charge carrier, suggesting that CaM binding motifs target a site outside of the pore region. We measured the combined effects of CBmp and Ca(2+)-CaM-dependent protein kinase II (CaMKII) on L-type Ca2+ channels by using an engineered Ca(2+)-CaM-independent form of CaMKII that remains active under low Ca(2+)-CaM conditions, permissive for CBmp signalling. CBmp and CaMKII increased L-type Ca2+ channel Po in a non-additive manner, suggesting that low and high Ca(2+)-CaM-dependent L-type Ca2+ channel facilitation pathways converge upon a common signalling mechanism.

Regulatory interaction of sodium channel IQ-motif with calmodulin C-terminal lobe

An increasing number of ion channels have been found to be regulated by the direct binding of calmodulin (CaM), but its structural features are mostly unknown. Previously, we identified the Ca(2+)-dependent and -independent interactions of CaM to the voltage-gated sodium channel via an IQ-motif sequence. In this study we used the trypsin-digested CaM fragments (TR(1)C and TR(2)C) to analyze the binding of Ca(2+)-CaM or Ca(2+)-free (apo) CaM with a sodium channel-derived IQ-motif peptide (NaIQ). Circular dichroic spectra showed that NaIQ peptide enhanced alpha-helicity of the CaM C-terminal lobe, but not that of the CaM N-terminal lobe in the absence of Ca(2+), whereas NaIQ enhanced the alpha-helicity of both the N- and C-terminal lobes in the presence of Ca(2+). Furthermore, the competitive binding experiment demonstrated that Ca(2+)-dependent CaM binding of target peptides (MLCKp or melittin) with CaM was markedly suppressed by NaIQ. The results suggest that IQ-motif sequences contribute to prevent target proteins from activation at low Ca(2+) concentrations and may explain a regulatory mechanism why highly Ca(2+)-sensitive target proteins are not activated in the cytoplasm.

A calmodulin/inositol 1,4,5-trisphosphate (IP3) receptor-binding region targets TRPC3 to the plasma membrane in a calmodulin/IP3 receptor-independent process

Conformational coupling with the inositol 1,4,5-trisphosphate (IP3) receptor has been suggested as a possible mechanism of activation of TRPC3 channels and a region in the C terminus of TRPC3 has been shown to interact with the IP3 receptor as well as calmodulin (calmodulin/IP3 receptor-binding (CIRB) region). Here we show that internal deletion of 20 amino acids corresponding to the highly conserved CIRB region results in the loss of diacylglycerol and agonist-mediated channel activation in HEK293 cells. By using confocal microscopy to examine the cellular localization of Topaz fluorescent protein fusion constructs, we demonstrate that this loss in activity is caused by faulty targeting of CIRB-deleted mutants to intracellular compartments. Wild type TRPC3 and mutants lacking a C-terminal predicted coiled coil region downstream of CIRB were targeted to the plasma membrane correctly in HEK293 cells and exhibited TRPC3-mediated calcium entry in response to agonist activation. Mutation of conserved YQ and MKR motifs to alanine within the CIRB region in TRPC3-Topaz, which would be expected to interfere with IP3 receptor and/or calmodulin binding, had no effect on channel function or targeting. Additionally, TRPC3 targets to the plasma membrane of DT40 cells lacking all three IP3 receptors and forms functional ion channels. These findings indicate that the previously identified CIRB region of TRPC3 is involved in its targeting to the plasma membrane by a mechanism that does not involve interaction with IP3 receptors.

Small conductance Ca2+-activated K+ channels and calmodulin: cell surface expression and gating

Small conductance Ca2+-activated K+ channels (SK channels) are heteromeric complexes of pore-forming alpha subunits and constitutively bound calmodulin (CaM). The binding of CaM is mediated in part by the electrostatic interaction between residues Arg-464 and Lys-467 of SK2 and Glu-84 and Glu-87 of CaM. Heterologous expression of the double charge reversal in SK2, SK2 R464E/K467E (SK2:64/67), did not yield detectable surface expression or channel activity in whole cell or inside-out patch recordings. Coexpression of SK2:64/67 with wild type CaM or CaM1,2,3,4, a mutant lacking the ability to bind Ca2+, rescued surface expression. In patches from cells coexpressing SK2:64/67 and wild type CaM, currents were recorded immediately following excision into Ca2+-containing solution but disappeared within minutes after excision or immediately upon exposure to Ca2+-free solution and were not reactivated upon reapplication of Ca2+-containing solution. Channel activity was restored by application of purified recombinant Ca2+-CaM or exposure to Ca2+-free CaM followed by application of Ca2+-containing solution. Coexpression of the double charge reversal E84R/E87K in CaM (CaM:84/87) with SK2:64/67 reconstituted stable Ca2+-dependent channel activity that was not lost with exposure to Ca2+-free solution. Therefore, Ca2+-independent interactions with CaM are required for surface expression of SK channels, whereas the constitutive association between the two channel subunits is not an essential requirement for gating.

FRET two-hybrid mapping reveals function and location of L-type Ca2+ channel CaM preassociation

L-type Ca(2+) channels possess a Ca(2+)-dependent inactivation (CDI) mechanism, affording feedback in diverse neurobiological settings and serving as prototype for unconventional calmodulin (CaM) regulation emerging in many Ca(2+) channels. Crucial to such regulation is the preassociation of Ca(2+)-free CaM (apoCaM) to channels, facilitating rapid triggering of CDI as Ca(2+)/CaM shifts to a channel IQ site (IQ). Progress has been hindered by controversy over the preassociation site, as identified by in vitro assays. Most critical has been the failure to resolve a functional signature of preassociation. Here, we deploy novel FRET assays in live cells to identify a 73 aa channel segment, containing IQ, as the critical preassociation pocket. IQ mutations disrupting preassociation revealed accelerated voltage-dependent inactivation (VDI) as the functional hallmark of channels lacking preassociated CaM. Hence, the alpha(1C) IQ segment is multifunctional-serving as ligand for preassociation and as Ca(2+)/CaM effector site for CDI.

TRPM5 is a voltage-modulated and Ca(2+)-activated monovalent selective cation channel

The TRPM subfamily of mammalian TRP channels displays unusually diverse activation mechanisms and selectivities. One member of this subfamily, TRPM5, functions in taste receptor cells and has been reported to be activated through G protein-coupled receptors linked to phospholipase C. However, the specific mechanisms regulating TRPM5 have not been described. Here, we demonstrate that TRPM5 is a monovalent-specific cation channel with a 23 pS unitary conductance. TRPM5 does not display constitutive activity. Rather, it is activated by stimulation of a receptor pathway coupled to phospholipase C and by IP(3)-mediated Ca(2+) release. Gating of TRPM5 was dependent on a rise in Ca(2+) because it was fully activated by Ca(2+). Unlike any previously described mammalian TRP channel, TRPM5 displayed voltage modulation and rapid activation and deactivation kinetics upon receptor stimulation. The most closely related protein, the Ca(2+)-activated monovalent-selective cation channel TRPM4b, also showed voltage modulation, although with slower relaxation kinetics than TRPM5. Taken together, the data demonstrate that TRPM5 and TRPM4b represent the first examples of voltage-modulated, Ca(2+)-activated, monovalent cation channels (VCAMs). The voltage modulation and rapid kinetics provide TRPM5 with an excellent set of properties for participating in signaling in taste receptors and other excitable cells.

Ionic mechanisms mediating oscillatory membrane potentials in wide-field retinal amacrine cells

Particular types of amacrine cells of the vertebrate retina show oscillatory membrane potentials (OMPs) in response to light stimulation. Historically it has been thought the oscillations arose as a result of circuit properties. In a previous study we found that in some amacrine cells, the ability to oscillate was an intrinsic property of the cell. Here we characterized the ionic mechanisms responsible for the oscillations in wide-field amacrine cells (WFACs) in an effort to better understand the functional properties of the cell. The OMPs were found to be calcium (Ca2+) dependent; blocking voltage-gated Ca2+ channels eliminated the oscillations, whereas elevating extracellular Ca2+ enhanced them. Strong intracellular Ca2+ buffering (10 mM EGTA or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid) eliminated any attenuation in the OMPs as well as a Ca2+-dependent inactivation of the voltage-gated Ca2+ channels. Pharmacological and immunohistochemical characterization revealed that WFACs express L- and N-type voltage-sensitive Ca2+ channels. Block of the L-type channels eliminated the OMPs, but omega-conotoxin GVIA did not, suggesting a different function for the N-type channels. The L-type channels in WFACs are functionally coupled to a set of calcium-dependent potassium (K(Ca)) channels to mediate OMPs. The initiation of OMPs depended on penitrem-A-sensitive (BK) K(Ca) channels, whereas their duration is under apamin-sensitive (SK) K(Ca) channel control. The Ca2+ current is essential to evoke the OMPs and triggering the K(Ca) currents, which here act as resonant currents, enhances the resonance as an amplifying current, influences the filtering characteristics of the cell membrane, and attenuates the OMPs via CDI of the L-type Ca2+ channel.

Proteomic analysis of human prostasomes

BACKGROUND

Prostasomes are secretory particles in human seminal fluid. Other than a microscopic description of these secretory particles and an incomplete two-dimensional gel electrophoresis (2DE) study, little is known about the composition of proteins in prostasomes.

METHODS

We employed a direct iterative approach using Gas phase fractionation and microcapillary HPLC-tandem mass spectrometry (microLC-MS/MS) to catalogue the prostasome proteome.

RESULTS

We identified 139 proteins that can be divided into the following categories: (1). enzymes (33.8% of total), (2). transport/structural (19.4% of total), (3). GTP proteins (14.4% of total), (4). chaperone proteins (5.8% of total), (5). signal transduction proteins (17.3% of total), and (6). unannotated proteins (9.4% of total). A total of 128 of the 139 proteins have not previously been described as prostasomal.

CONCLUSIONS

The proteins identified can be used as reference dataset in future work comparing prostasome proteins between normal and pathological states such as prostate cancer, benign prostatic hyperplasia, prostatitis, and infertility.